Drive circuit for switching device

ABSTRACT

A drive circuit that controls a switching device ON/OFF and a soft cutoff command circuit that gradually decreases the gate terminal voltage of the switching device when short circuit of the switching device is detected. Additionally, an ON-pulse retention command circuit retains the output of the drive circuit ON when the gate terminal voltage is judged to have exceeded a specified value by a gate voltage judgment comparator that detects the gate terminal voltage of the switching device.

This application is a continuation of U.S. Ser. No. 10/784,161, filedFeb. 26, 2004 now U.S. Pat. No. 7,126,802, which claims priority to JP2003-165579, filed Jun. 10, 2003, the contents of each of which areincorporated by reference herein in their entirety.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to a drive circuit for a voltage drivenswitching device.

When a semiconductor switching device, such as insulated gate typebipolar transistor (IGBT), is used for driving a motor, and if arm shortcircuit or short circuit of load is caused, the collector current of theIGBT increases quickly and consequently the IGBT is broken down due toovercurrent or heat. It, therefore, is necessary to detect a shortcircuit state in a short time. When classifying roughly, there are twomajor known methods of detecting the short circuit. One is a method, asdisclosed in the Japanese Application Patent Laid-open Publication No.Hei 04-79758, where a current sense IGTB is used to detect the shortcircuit current. The other is a method, as disclosed in the JapaneseApplication Patent Laid-open Publication No. Hei 02-262826, where thecollector voltage is monitored to detect the short circuit state.

On the other hand, there remains a problem that, if current is cut offquickly in a short circuit state, the rate of current change—dI/dtbecomes greater and consequently the spike voltage, which is defined bythe product of the rate and the stray inductance, becomes greater,resulting in the breakdown of the IGBT. To prevent this, the methoddisclosed in the Japanese Application Patent Laid-Open Publication Nos.Hei 04-79758 and 02-262826 is provided with a soft cutoff function so asto cut off the main IGBT slowing in the case short circuit is detected.

SUMMARY OF THE INVENTION

According to the method disclosed in the Japanese Application PatentLaid-open Publication No. Hei 04-79758 and 02-262826, however, becausethe short circuit detection circuit is provided with a delay time forpreventing malfunction due to noise, there is a time delay of 1 to 2 μsbefore the soft cutoff function operates. Because of the above, thereremains a problem that, if a turn-OFF command signal is inputted duringthe delay time of 1 to 2 μs before the soft cutoff function operates,the main circuit current is cut off quickly and that the rate of currentchange —dI/dt at the cutoff becomes greater and consequently the spikevoltage, defined by the product of the rate and the stray inductance,becomes greater, resulting in the breakdown of the IGBT. An example ofthe above explanation that a turn-OFF command signal is inputted duringthe delay time of 1 to 2 μs before the soft cutoff function operates canbe a case where noise superimposes a PWM signal. Because noise is anarrow pulse of about 1 μs, an end portion of the noise may beincorrectly recognized as a turn-OFF signal.

An object of the present invention is to offer such drive circuit for aswitching device that can operate a soft cutoff function when shortcircuit is detected and also can be prevented from breakdown even if anarrow pulse is inputted.

(1) In order to realize the above object, the present inventionconstructs a drive circuit for a switching device, comprising a drivecircuit that controls the switching device ON/OFF, a short circuitdetection means that detects short circuit of the switching device, anda soft cutoff means that gradually decreases the gate terminal voltageof the switching device when short circuit is detected by the shortcircuit detection means; equipped with a gate voltage detection meansthat detects the gate terminal voltage of the switching device, and anON-pulse retention means that retains the drive circuit output ON whenthe gate terminal voltage detected by the gate voltage detection meansexceeds a specified value.

With this construction, it becomes possible to operate the soft cutofffunction when short circuit is detected and also to prevent breakdowneven if a narrow pulse is inputted while the drive circuit output isretained ON.

(2) In (1) above, it is preferable that the drive circuit furthercomprises a gate voltage clamp means that clamps the gate voltage of theswitching device, wherein the gate voltage clamp means is operated by anoutput signal of the ON-pulse retention means.

(3) In (1) above, it is preferable that the switching device is one withvoltage driven sense function, which is provided with a gate terminal,terminal No. 1, terminal No. 2, and terminal No. 3, where the maincurrent is applied between the terminal No. 1 and terminal No. 2 and thesense current in proportion to the main current is applied between theterminal No. 1 and terminal No. 3 by applying voltage to the gateterminal, and the ON-pulse retention means retains the drive circuitoutput ON when the gate terminal voltage detected by the gate voltagedetection means exceeds a specified value and also the sense voltage ofa sense resistor, connected in series to the terminal No. 3 of theswitching device, exceeds a specified value.

(4) In (1) above, it is preferable that the drive circuit furthercomprises a pulse width extension means that extends the pulse width ofa pulse signal inputted to the drive circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is circuit block diagram showing the construction of the drivecircuit of a switching device according to the first embodiment of theinvention

FIG. 2 is time chart showing the operation of the drive circuit of aswitching device according to the first embodiment of the invention

FIG. 3 is circuit block diagram showing the construction of the drivecircuit of a switching device according to the second embodiment of theinvention

FIG. 4 is time chart showing the operation of the drive circuit of aswitching device according to the second embodiment of the invention

FIG. 5 is circuit block diagram showing the construction of the drivecircuit of a switching device according to the third embodiment of theinvention

FIG. 6 is circuit block diagram showing the construction of the drivecircuit of a switching device according to the fourth embodiment of theinvention

FIG. 7 is circuit block diagram showing the construction of the drivecircuit of a switching device according to the fifth embodiment of theinvention

FIG. 8 is circuit block diagram showing the construction of the drivecircuit of a switching device according to the sixth embodiment of theinvention

FIG. 9 is circuit block diagram showing the construction of the drivecircuit of a switching device according to the seventh embodiment of theinvention

FIG. 10 is time chart showing the operation of the drive circuit of aswitching device according to the seventh embodiment of the invention

DETAILED DESCRIPTION OF THE INVENTION

The construction and operation of the drive circuit of a switchingdevice according to the first embodiment of the invention are describedhereunder, using FIG. 1 and FIG. 2.

FIG. 1 is a circuit block diagram showing the construction of the drivecircuit of the switching device according to the first embodiment of theinvention.

In this embodiment, IGBT 23 with sense function is used as a switchingdevice. In the IGBT 23, the main current can be applied between theterminal No. 1 (collector terminal) and terminal No. 2 (emitterterminal) and the sense current, which is one-hundredths toone-thousandths of the main current, can be applied between the terminalNo. 1 (collector terminal) and terminal No. 3 (sense emitter terminal).The drive circuit 21A is connected to the gate terminal of the IGBTthrough gate resistor Rg, and the drive circuit 21 has a function ofsetting the IGBT 23 ON/OFF when a PWM input signal is transmitted.

Besides, a sense resistor Rs is connected to the terminal No. 3 (emitterterminal) in series with the ground. A short circuit judgment comparator27 measures the voltage of the sense resistor Rs and judges asshort-circuited when the voltage of the sense resistor Rs is higher thana reference voltage (sense voltage detection level) VSD. When the shortcircuit judgment comparator 27 judges as short-circuited, a soft cutoffcommand circuit 28 stops setting the drive circuit 21 ON/OFF, sets thenMOS 29 for soft cutoff ON, and also decreases the gate voltagegradually by a resistor Rsf for soft cutoff so as to cut off the currentthrough the IGBT slowly. Because of this, the spike voltage defined bythe product of the rate of current change—dI/dt at the cutoff and thestray inductance can be lowered.

A high-frequency noise is generated in the voltage of the sense resistorRs due to noise of switching or because of lengthy wiring up to theshort circuit judgment comparator 27. There is, therefore, provided afilter circuit 26 for eliminating the noise between the sense resistorRs and short circuit judgment comparator 27. The filter circuit 26 has acharacteristic that the output voltage rises slowly with a delay of 1 to2 μs even if the input voltage, which is the voltage of the senseresistor Rs, rises quickly.

When a 10 kHz signal is employed as the PWM signal, one cycle of the PWMsignal is 100 μs. Because the on-duty minimum of the PWM signal isgenerally set to 2 to 3%, the minimum pulse width of the ON-pulse of thePWM signal is 2 to 3 μs. Adding the filter circuit 26, therefore, doesnot cause any problem because, when a PWM signal of 2 to 3 μs isinputted, the delay of the soft cutoff command is no more than 1 to 2μs. If a narrow signal of 1 to 2 μs, such as a noise signal, isinputted, however, the OFF command signal of the PWM and the soft cutoffcommand signal compete with each other and the IGBT 23 may bebroken-down.

This embodiment, therefore, are provided with a gate voltage judgmentcomparator 16 and an ON-pulse retention command circuit 11 as acountermeasure against narrow pulse. An ON-pulse retention signaloutputted from the ON-pulse retention command circuit 11 is inputted tothe drive circuit 21 via an OR circuit 22.

If arm short circuit is caused, the gate voltage is charged in excess ofthe supply voltage and consequently current through the IGBT quicklyincreases as the gate voltage increases. For this reason, measuring thegate voltage is the earliest way to judge whether the device isshort-circuited or not. The gate voltage judgment comparator 16 is acircuit that monitors the gate voltage. When the gate voltage exceedsthe reference voltage (gate voltage detection level) VGD, the gatevoltage judgment comparator 16 judges arm short circuit has been caused.For planar IGBT, the gate voltage detection level VGD is setapproximately to the supply voltage (15V) of the drive circuit. Fortrench gate IGBT, however, because the saturation current is extremelyhigh, it is set to a level lower than the supply voltage. For example,if the threshold of the IGBT is about 6V, the level is set to about 14Vfor planar IGBT, and to about 9V for trench gate IGBT. In short, it ispreferable that the level be set to 9 to 14V.

Next, the operation of the drive circuit of the switching deviceaccording to this embodiment is described, using FIG. 2.

FIG. 2 is a time chart showing the operation of the drive circuit of theswitching device according to the first embodiment of the invention.FIG. 2 shows the operation in the case where arm short circuit is causedwhen a narrow pulse is inputted. The horizontal axis of FIG. 2represents the time. FIG. 2 (A) represents a PWM signal inputted to thedrive circuit 21. FIG. 2 (B) represents an ON-pulse retention signaloutputted from the ON-pulse retention command circuit 11. FIG. 2 (C)represents an output signal of the drive circuit 21. FIG. 2 (D)represents the gate voltage of the IGBT 23. FIG. 2 (E) represents themain IGBT current through the IGBT 23. FIG. 2 (F) represents the sensevoltage, an output of the filter circuit 26.

It is assumed that, as shown in FIG. 2 (A), a narrow pulse due to noiseis inputted as a PWM signal. It is also assumed that the PWM signal isset ON at the trailing edge and set OFF at the leading edge of the pulsesignal. In other words, the PWM signal is set ON at time t1 and set OFFat time t4. The pulse width ΔT1 of the PWM signal is set, for example,to 1 μs. Since the minimum ON-time of normal PWM signal is set to 2 to 3μs, a narrow pulse of 1 μs is nothing but one generated by noise orsomething like that.

While the PWM signal is inputted to the drive circuit 21 via the ORcircuit 22, the drive circuit 21 is provided with a delay time forturn-ON and turn-OFF. That is, when the PWM signal falls at time t1 asshown in FIG. 2 (A), the output signal of the drive circuit 21 delays bytime ΔT2 and turns ON at time t2 as shown in FIG. 2 (C). When the PWMsignal rises at time t4 as shown in FIG. 2 (A), the output signal of thedrive circuit 21 delays by time ΔT3 and turns OFF. The delay time ΔT2and ΔT3 for turn-ON and turn-OFF is, for example, 0.5 μs.

When the output signal of the drive circuit 21 turns ON at time t2 asshown in FIG. 2 (C), the gate voltage of the IBGT begins increasing attime t2 as shown in FIG. 2 (D).

If arm short circuit is caused during the above operation, the gatevoltage is charged in excess of the supply voltage as shown in FIG. 2(D). Consequently, the gate voltage exceeds the gate voltage detectionlevel VGD. As the gate voltage increases, current through the IGBT alsoincreases quickly as shown in FIG. 2 (E). Because of the delay action ofthe filter 23 for noise prevention, the sense voltage, which is theoutput of the filter 23, increases with delay as shown in FIG. 2 (F).

If arm short circuit is caused, since the gate voltage exceeds the gatevoltage detection level VGD at time t3 as shown in FIG. 2 (D), the gatevoltage judgment comparator 16 detects as short-circuited and theON-pulse retention command circuit 11 outputs an ON-pulse retentionsignal at time t3 as shown in FIG. 2 (B). A one-shot multi-vibrator, forexample, can be employed as the ON-pulse retention command circuit 11and the pulse width ΔT4 of the ON-pulse retention signal is set, forexample, to 2 to 3 μs.

The drive circuit 21 determines the output signal shown in FIG. 2 (C)based on the logical OR of the PWM signal shown in FIG. 2 (A) and theON-pulse retention signal shown in FIG. 2 (B). The delay time ΔT2 andΔT3 for turn-ON and turn-OFF is as mentioned above. When the ON-pulseretention signal turns OFF at time t7, the drive circuit output signalturns OFF at time t8. Because of this, as shown in FIG. 2 (C), the drivecircuit 21 continues outputting the ON-pulse even after time, which isthe time the turn-OFF delay time has elapsed since the PWM signal turnedOFF at time t4. Accordingly, as shown in FIG. 2 (D), the gate voltagedoes not go OFF even after time t5.

If the sense voltage exceeds the sense voltage detection level VSD, forexample, at time t6 as shown in FIG. 2 (F), the short circuit judgmentcomparator 27 judges as short-circuited. Then, the soft cutoff commandcircuit 28 stops setting the drive circuit 21 ON/OFF, sets the nMOS 29for soft cutoff ON, and decreases the gate voltage gradually by aresistor Rsf for soft cutoff so as to cut off the current trough theIGBT slowly as shown in FIG. 2 (E). Because of this, the spike voltagedefined by the product of the rate of current change—dI/dt at the cutoffand the stray inductance can be lowered.

Thus, providing the gate voltage judgment comparator 16 and ON-pulseretention command circuit 11 makes it possible to surely protect thedevice from short circuit even if a narrow pulse is inputted upon shortcircuit.

Now, brief explanation is given hereunder on a case that no ON-pulseretention command circuit 11 is provided. If no ON-pulse retentioncommand circuit 11 is provided, assuming that a narrow pulse falls attime t1 and rises at time t4 as shown in FIG. 2 (A), the output signalof the drive circuit is set ON at time t2 and set OFF at time t5 asshown in FIG. 2 (C). On the other hand, if the sense voltage exceeds thelevel VSD as shown in FIG. 2 (F) and short circuit to the ground isdetected, the soft cutoff function operates.

This means that the turn-OFF command is inputted at time t5 before thesoft cutoff command is inputted. Consequently, because the gate voltageis decreased at a normal switching speed, the gate voltage cuts off themain circuit current at high speed. Thus, the spike voltage defined bythe product of the rate of current change—dI/dt at the cutoff and thestray inductance increases and the IGBT may be broken-down.

Although the ON-time ΔT4 of the ON-pulse retention signal outputted fromthe ON-pulse retention command circuit 11 is set to 2 to 3 μs, it can beanything shorter than the short circuit durability of the IGBT 23. Forexample, if the short circuit durability of the IGBT is 10 μs, theON-time must be shorter than this. Although the gate voltage is retainedat a high level during the time while the ON-pulse is retained, the IGBTcan be prevented from breakdown if the time is shorter than the shortcircuit durability and the soft cutoff function operates before theshort circuit durability time has elapsed.

As described above, by retaining the ON-pulse, this embodiment enablesto surely protect the device from short circuit even if a narrow pulseis inputted upon short circuit.

Next, the construction and operation of the drive circuit of a switchingdevice according to the second embodiment of the invention aredescribed, using FIG. 3 and FIG. 4.

To begin with, the construction and operation of the drive circuitswitching device according to this embodiment is described, using FIG.3.

FIG. 3 is a circuit block diagram showing the construction of the drivecircuit of the switching device according to the second embodiment ofthe invention. The same symbol as in FIG. 1 denotes the same part.

In this embodiment, in addition to the construction shown in FIG. 1,there is provided a voltage clamp circuit 18, comprising a gate voltageclamp nMOS 17 and a gate voltage clamp resistor Rcr. When the gate clampnMOS 17 is set ON, the gate voltage VG of the IGBT 23 is retained at avoltage (VDR×(Rcr/(Rcr+Rg)), which is the output voltage VDR of thedrive circuit divided by the resistor Rg and resistor RCR. Given thatthe output voltage VDR of the drive circuit 21 is 15V, the resistance ofthe gate voltage clamp resistor Rcr is so set that the clamp voltage is10 to 12V. Besides, the clamp voltage is set lower than the gate voltagedetection level VGD.

Next, the operation of the drive circuit of the switching deviceaccording to this embodiment is described, using FIG. 4.

FIG. 4 is a time chart showing the operation of the drive circuit of theswitching device according to the second embodiment of the invention.FIG. 4 shows the operation in the case where arm short circuit is causedwhen a narrow pulse is inputted. The horizontal axis of FIG. 4represents the time. FIG. 4 (A) represents a PWM signal inputted to thedrive circuit 21. FIG. 4 (B) represents an ON-pulse retention signaloutputted from the ON-pulse retention command circuit 11. FIG. 4 (C)represents an output signal of the drive circuit 21. FIG. 4 (D)represents the gate voltage of the IGBT 23. In FIG. 4 (D), broken linerepresents the gate voltage in the case of using no gate voltage clampcircuit 18, which is equivalent to FIG. 2 (D). Bold line in FIG. 4 (D)represents the gate voltage in the case where the clamp circuit 18functions. FIG. 4 (E) represents the main IGBT current through the IGBT23. FIG. 2 (F) represents the sense voltage, an output of the filtercircuit 26.

If arm short circuit is caused, the gate voltage exceeds the gatevoltage detection level as shown in FIG. 4 (F), the gate voltagejudgment comparator 16 judges as short-circuited, and an ON-pulseretention signal is outputted as shown in FIG. 4 (B). The drive circuit21 determines the drive circuit output signal based on the logical OR ofthe PWM signal and ON-pulse retention signal. Consequently, as shown inFIG. 4 (C), it continues outputting the ON-pulse even after the PWMsignal turns to an OFF command.

In this embodiment, the gate voltage clamp circuit 18 is providedadditionally, and hence the gate voltage is lowered during the ON-pulsesignal as shown by bold line in FIG. 4 (D). Because of this, the shortcircuit current of the IGBT can be decreased as shown by bold line inFIG. 4 (E).

When a trench gate IGBT is used, the saturation current is about tentimes as high as the rated current. With the above method, however, thecurrent through the IGBT upon short circuit can be decreased to about ahalf.

In this embodiment, because the gate voltage clamp circuit 18 isprovided in addition to the gate voltage judgment comparator 16 andON-pulse retention command circuit 11 as described above, it becomespossible even in the case of short circuit caused by a short PWM signalinput not only to surely protect the device from short circuit but alsoto decrease the short circuit current. Accordingly, this embodimentfurther improves the safety as compared to the embodiment shown in FIG.1.

Next, the construction and operation of the drive circuit of a switchingdevice according to the third embodiment of the invention are described,using FIG. 5.

FIG. 5 is a circuit block diagram showing the construction of the drivecircuit of the switching device according to the third embodiment of theinvention. The same symbol as in FIG. 1 denotes the same part.

In this embodiment, an IGBT 24 is used in place of the IGBT 23 withsense function that is used in the embodiment shown in FIG. 1. While, inthe embodiment shown in FIG. 1, the sense current is detected for theprotection of the device from short circuit, a short circuit judgmentcomparator 27A in this embodiment monitors the anode voltage of ahigh-voltage diode 31 and judges as short-circuited if it is higher thana specified voltage VAD.

This embodiment is also provided with a gate voltage judgment comparator16 and an ON-pulse retention command circuit 11. If arm short circuit iscaused when a narrow pulse signal is inputted, the gate voltage exceedsthe gate voltage detection level, the gate voltage judgment comparator16 detects short circuit, and the ON-pulse retention command circuitoperates. The drive circuit 21 determines the drive circuit outputsignal based on the logical OR of the PWM signal and ON-pulse retentionsignal. Consequently, it continues outputting the ON-pulse even afterthe PWM signal turns to an OFF command. Then, the short circuit judgmentcomparator 27A judges as short-circuited, and the soft cutoff commandcircuit 28 stops setting the drive circuit 21 ON/OFF, sets the nMOS 29for soft cutoff ON, and decreases the gate voltage gradually by aresistor Rsf for soft cutoff. The current trough the IGBT, therefore,can be cut off slowly and the spike voltage can be lowered.

With this embodiment, even in the case of short circuit caused by anarrow pulse input, the device can be protected from short circuit byretaining the ON-pulse as described above.

Next, the construction and operation of the drive circuit of a switchingdevice according to the fourth embodiment of the invention aredescribed, using FIG. 6.

FIG. 6 is a circuit block diagram showing the construction of the drivecircuit of the switching device according to the fourth embodiment ofthe invention. The same symbol as in FIG. 5 denotes the same part.

In this embodiment, in addition to the gate voltage judgment comparator16 and ON-pulse retention command circuit 11 in the embodiment shown inFIG. 5, there is provided a voltage clamp circuit 18, comprising a gatevoltage clamp nMOS 17 and a gate voltage clamp resistor Rcr. Theconstruction and operation of the voltage clamp circuit 18 are the sameas explained for FIG. 3.

If arm short circuit is caused by a narrow pulse signal input, the gatevoltage exceeds the gate voltage detection level, the gate voltagejudgment comparator 16 judges as short-circuited and an ON-pulseretention signal functions. The drive circuit 21 determines the drivecircuit output signal based on the logical OR of the PWM signal andON-pulse retention signal. Consequently, it continues outputting theON-pulse even after the PWM signal turns to an OFF command. In addition,in this embodiment, by providing the gate voltage clamp circuit 18, thegate voltage is lowered during the ON-pulse retention signal. Because ofthis, the short circuit current of the IGBT can be decreased.

With this embodiment, it becomes possible not only to protect the devicefrom short circuit caused by a short PWM signal input but also todecrease the short circuit current, and hence improve the safety.

Next, the construction and operation of the drive circuit of a switchingdevice according to the fifth embodiment of the invention are described,using FIG. 7.

FIG. 7 is a circuit block diagram showing the construction of the drivecircuit of the switching device according to the fifth embodiment of theinvention. The same symbol as in FIG. 1 denotes the same part.

In this embodiment, sense resistors Rs1 and Rs2 are connected in serieswith the ground. The voltage of these sense resistors Rs1 and Rs2 aremeasured for a short circuit judgment comparator 27 to judge whether thedevice is short-circuited or not. In addition, there are provided asense voltage judgment comparator 19 and an AND circuit 20. The sensevoltage judgment comparator 19 measures the voltage of the senseresistor Rs1 and judges as short-circuited if it is higher than aspecified voltage VSD2.

In this embodiment, therefore, whether both sense voltage judgmentcomparator 19 and gate voltage judgment comparator 16 have detectedshort circuit is judged by the AND circuit 20, and if so judged, theON-pulse retention command signal circuit 11 operates, and the ON-pulseretention signal is outputted. Because of monitoring the IGBT currentand gate voltage at the same time as well as retaining the ON-pulse, itbecomes possible even in the case of short circuit caused by a narrowpulse signal input to further surely protect the device from shortcircuit.

Next, the construction and operation of the drive circuit of a switchingdevice according to the sixth embodiment of the invention are described,using FIG. 8.

FIG. 8 is a circuit block diagram showing the construction of the drivecircuit of the switching device according to the fifth embodiment of theinvention. The same symbol as in FIG. 7 denotes the same part.

In this embodiment, in addition to the gate voltage judgment comparator16 and ON-pulse retention command circuit 11 in the embodiment shown inFIG. 7, there is provided a voltage clamp circuit 18. The constructionand operation of the voltage clamp circuit 18 are the same as explainedfor FIG. 3.

With this embodiment, by lowering the gate voltage during the ON-pulseretention signal, the short circuit current through the IGBT can also bedecreased. Accordingly, it becomes possible not only to protect thedevice from short circuit caused by a narrow pulse signal input but alsoto decrease the short circuit current.

Next, the construction and operation of the drive circuit of a switchingdevice according to the seventh embodiment of the invention aredescribed, using FIG. 9 and FIG. 10.

To begin with, the construction of the drive circuit of a switchingdevice according to the seventh embodiment of the invention isdescribed, using FIG. 9.

FIG. 9 is a circuit block diagram showing the construction of the drivecircuit of the switching device according to the seventh embodiment ofthe invention. The same symbol as in FIG. 1 denotes the same part.

In this embodiment, there is provided a narrow pulse extension circuit42 in addition to the construction shown in FIG. 1. The narrow pulseextension circuit 42 is to extend the pulse width of a short pulsesignal, for example, shorter than 1 μs, to a longer width than 1 μs. Thenarrow pulse extension circuit 42 can be composed, for example, using acomparator.

Next, the operation of the drive circuit of the switching deviceaccording to this embodiment is described, using FIG. 10.

FIG. 10 is a time chart showing the operation of the drive circuit ofthe switching device according to the seventh embodiment of theinvention. FIG. 10 shows the operation in the case where arm shortcircuit is caused when a narrow pulse is inputted. The horizontal axisof FIG. 10 represents the time. FIG. 10 (A) represents a PWM signalinputted to the drive circuit 21. FIG. 10 (A′) represents a PWM signaloutputted from the narrow pulse extension circuit 42. FIG. 10 (B)represents an ON-pulse retention signal outputted from the ON-pulseretention command circuit 11. FIG. 10 (C) represents an output signal ofthe drive circuit 21. FIG. 10 (D) represents the gate voltage of theIGBT 23. FIG. 10 (E) represents the main IGBT current through the IGBT23. FIG. 10 (F) represents the sense voltage, an output of the filtercircuit 26.

When a narrow pulse having the pulse width of ΔT5 is inputted at timet11 as shown in FIG. 10 (A), the narrow pulse extension circuit 42extends the pulse width to ΔT6 (>ΔT5) and outputs it at time T13 asshown in FIG. 10 (A′).

The output signal of the drive circuit 21 is made based on the logicalOR of the PWM signal shown in FIG. 10 (A′) and the ON-pulse retentionsignal shown in FIG. 10 (B), and the drive circuit output signal is setON at time t13 as shown in FIG. 10 (C).

If arm short circuit is caused, the gate voltage exceeds the gatevoltage detection level as shown in FIG. 10 (D), the gate voltagejudgment comparator 16 judges as short-circuited, and an ON-pulseretention signal is outputted as shown in FIG. 10 (B). The drive circuit21 determines the drive circuit output signal based on the logical OR ofthe PWM signal and ON-pulse retention signal. Consequently, as shown inFIG. 10 (C), it continues outputting the ON-pulse even after the PWMsignal turns to an OFF command.

If the sense voltage exceeds the sense voltage detection level VSD, forexample, at time t15 as shown in FIG. 10 (F), the short circuit judgmentcomparator 27 judges as short-circuited. Then, the soft cutoff commandcircuit 28 stops setting the drive circuit 21 ON/OFF, sets the nMOS 29for soft cutoff ON, and decreases the gate voltage gradually by aresistor Rsf for soft cutoff. Because of this, the current trough theIGBT can be cut off slowly and the spike voltage defined by the productof the rate of current change—dI/dt at the cutoff and the strayinductance can be lowered as shown in FIG. 10 (E).

Thus, providing the gate voltage judgment comparator 16 and ON-pulseretention command circuit 11 makes it possible to surely protect thedevice from short circuit in the case of short circuit caused by anarrow pulse input.

In addition, providing the narrow pulse extension circuit 42 makes itpossible to further improve the protection from short circuit in thecase of a PWM signal having much narrower pulse width as shown in FIG.10 (A). For example, in the case where the PWM signal rises at time t11and falls at time t12 as shown in FIG. 10 (A), if the output signal ofthe drive circuit 21 is set ON at time t13 as shown in FIG. 10 (C) andset OFF at time later than time t13 but earlier than time t14 at whichthe ON-pulse retention signal is outputted, quick cutoff may possibly becaused to the IGBT because the ON-pulse retention signal has not yetbeen set ON by this time. Even in the above case, by extending the pulsewidth by the narrow pulse extension circuit 42, the drive circuit outputsignal can continue being ON, and hence the IGBT can be protected frombreakdown.

With this embodiment, therefore, it is possible even in the case ofshort circuit caused by a narrower pulse signal input to further surelyprotect the device from short circuit.

According to the present invention, it becomes possible to operate asoft cutoff function when short circuit is detected and also to preventthe device from breakdown even if a narrow pulse is inputted.

1. A drive circuit for controlling a switching device ON/OFF,comprising: a short circuit detection means that detects short circuitof the switching device; a soft cutoff means that performs a soft-cutofffunction by gradually decreasing the gate terminal voltage of theswitching device when short circuit is detected by the short circuitdetection means; wherein the switching device is one with voltage drivensense function, which is provided with a gate terminal, terminal No. 1,terminal No. 2, and terminal No. 3, with the main current appliedbetween the terminal No. 1 and terminal No. 2 and the sense current inproportion to the main current is applied between the terminal No. 1 andterminal No. 3 by applying voltage to the gate terminal, a gate voltagedetection means that detects the gate terminal voltage of the switchingdevice; and an ON-pulse retention means that retains the drive circuitoutput ON when the gate terminal voltage detected by the gate voltagedetection means exceeds a specified value and also the sense current ofthe switching device exceeds a specified value so that the soft cutofffunction continues to operate when the short circuit is detected whilepreventing breakdown when a narrow pulse is inputted.
 2. The drivecircuit for a switching device according to claim 1, further comprising:a pulse width extension means that extends the pulse width of a pulsesignal inputted to the drive circuit.